Cell membrane battery resting potential
The Cell’s Resting “Battery” Voltage
(or Membrane Resting Potential)
The cell’s resting “battery” voltage represents the health of the cell
“driving-force” for actively transporting ingredients required for cellular
energy production across the cell membrane -
This energy is
then used to create a driving force across its mitochondrial membranes for
“oxidative-phosphorylation” - the final, and most energy-yielding step, in the
mitochondrion’s energy production process –
Affects All Electrical
activity of the cell –
resting potential prepares the “excitable” nerve and muscle cells for the
propagation of action potentials leading to nerve impulses and muscle
contraction. E.g. the heart muscle cells (myocardial cells) require a sufficient
membrane potential for the heart to beat; pain messages are passed via nerve
The membrane potential
voltage controls the opening and closing of the potassium and sodium gates -
Most ion channels are “gated” - stimulated
to open or close by electrical (and sometimes mechanical or chemical)
mechanisms. Stimulated by the membrane potential, the opening of the Na and K
gates generates an inward current that affects the membrane potential itself
(creating a reinforcing positive loop). Thus, the membrane potential controls
the concentration and charge gradient of potassium and sodium ions either side
of the membrane.
“Resting” term implies, the cell is actually very busy keeping the unequal
distribution of ions on each side of the cell membrane
This is to establish a healthy cell membrane potential difference of about 70mV
across the membrane, with the
inside of the cell being more negative
than the outside
The concentration gradients of K, Na, Cl determine
the overall electrochemical gradient across the cell membrane
The concentration gradient of an ion is
determined by the difference in concentrations (shown in diagrams in mM, which
mol/m3) of the ions in solution on each
side of the membrane. Those concentrations have been converted here to their
equivalent electrical gradient in millivolts (mV).
The largest net
electrochemical gradient is 130mV, for Na+ ions into the cell
There is a small gradient of 20mV for K+ ions out of the cell
There is no gradient for Cl- ions
therefore, this should mean that more Na+ ions diffuse
into the cell than K+ ions diffuse out across the resting membrane.
actually, this is not so, because
we must also consider the
membrane permeability, which is
about 50 times more permeable to K+
Potassium can cross the membrane much more easily than Sodium).
The membrane’s permeability to an ion refers to the ability of its
membrane channels to conduct ions once they are open. This usually
depends on the size of the ions in solution compared to the size of the
channels. Most channels are large enough to pass the small
hydrated K+ ion, but few will carry the larger hydrated Na+
ion across the membrane.
How the Resting Membrane Potential is Produced
The cell’s resting
“battery” voltage is established by the
following 3 factors:
The Preference of the Cell Membrane for Potassium
to travel through it –
K+ ions diffuse out of the cell than Na+ ions diffuse into the cell.
Since the membrane
is much more permeable to K+ ions, there will be more K+ ions outside the
membrane than can be compensated by the inflow of Na+ ions.
Negatively Charged Molecules
are Trapped inside the Cell -
charged Molecules (here referred to as
) exist in the cytosol, which because of their molecular size,
insolubility or bound position in the cytosol, cannot migrate across the
membrane. Examples of these include proteins, DNA, RNA and
ATP, all behaving as organic acids,
giving off a hydrogen ion. When this positive ion is incorporated with oxygen to
form water, it leaves the negative ion inside the cell. Cumulatively, these
negative ions establish a negative charge trapped inside the cell.
The negative anions trapped inside the cell, which cannot follow the K+
ions across the membrane, together with the excess of K+ ions diffused out of
the cell over Na+ ions diffused into the cell, establish the resting potential.
The inside of
the cell has a negative charge
Sodium / Potassium Pumps -
Even though the cell membrane is not nearly as
permeable to sodium as potassium, over time the
slow diffusion of Na+ ions
into the cell, together with the fast diffusion of K+ ions out of the cell will
cause the cell membrane to lose its
potential voltage by equalizing the charges on either side of the membrane.
Eventually the cell membrane potential would drop to zero.
i.e. the cell membrane “battery” would be dead!
Fortunately, the cell
has a mechanism to bring back in the K+ ions and move out the Na+ ions - This mechanism is provided by the many
energy-using Na/K pumps spanning the cell membrane, which continuously pump
Na+ ions back out of and K+ ions back into the cell, against their
The Na/K Pump
energy to simultaneously pump:
Na+ ions out of the
into the cell.
The Na/K Pump thus counters the potassium /
sodium diffusion (leaking) out of the cell, and
increases the membrane potential by pumping out of the cell one
more sodium ion than it pumps potassium ions into the cell. The Na/K pump thus
provides a mechanism by which Na+
ions are driven out of the cell faster than
K+ ions are pulled in, keeping intracellular sodium
levels low and potassium levels high.
This accomplishes several vital functions:
It establishes an electrical potential difference (a “battery”) across
the cell membrane -
with the interior of the cell being negatively
charged relative to the exterior.
Maintains osmotic balance - Sodium ions accumulated outside of the cell
draw water out of the cell
(otherwise, the cell would swell and burst from the inward diffusion of water).
Provides energy for indirect pumps –
gradient of sodium ions. E.g. to transport glucose into the cell.
Operating these continuously active Na/K pumps
uses about 30% of the total ATP energy produced by the cell
The Mitochondrial Cell Membrane “Battery”
The mitochondrial membrane uses positively
charged hydrogen ions
(Protons or H+)
a strong membrane potential difference across the mitochondrial membrane -Hydrogen ions are
maintained in a high concentration on the outside of the mitochondrial membrane
by the action of the electron transport chain (See
Cellular Respiration), which creates a mitochondrial membrane potential of about
When this proton electricity flows back across
the inner mitochondrial membrane it is used to power a molecular, enzyme motor
called ATP syntase - which loads
negatively charged phosphate anions onto ADP thus creating